The production of antibiotic U-43,120 by Streptomyces paulus, NRRL 8115, and the physicochemical properties of this compound are described in U.S. Pat. No. 3,988,441 (Oct. 26, 1976). Recent work by us shows, unexpectedly, that antibiotic U-43,120, which we have now named paulomycin, is a mixture of two chemically related antibiotics. We have named these two new antibiotic entities paulomycin A and paulomycin B.